Sound speed (red) and density (blue) in the upper layers of the Sun as functions of height measured from the photosphere. Below 500 km (vertical dashed line), the sound speed and density are from model S (Christensen-Dalsgaard et al. 1996). Above 500 km, the model Atmo (solid lines) consists of an extension where density decays exponentially (H = 105 km) and sound speed is constant (c = 6855 m/s). The dashed lines show the VAL-C atmospheric model up to a height of 2500 km (Vernazza et al. 1981), which will be used in Sect. 6.